Bug 318740 – [Site Isolation] Web Inspector: Network domain sourceMappingURL is not forwarded for memory-cached cross-origin iframe stylesheets

• Status: RESOLVED
• Resolution: FIXED
• Component: Web Inspector

[Qianlang Chen]

Follow-up to Bug 310251 (rdar://172894304), which forwards the CSS sourceMappingURL for cross-origin iframe stylesheets under Site Isolation. That change only covers the cold-load path; stylesheets served from WebCore's in-memory cache are not covered, so their source maps stop auto-linking in the Network panel.

On the memory-cache path, the sourceMappingURL is neither computed nor delivered:

1. The header capture in BackendResourceDataStore::responseReceived never runs (that hook is not called for memory-cached resources), so no header-derived sourceMappingURL is stored.

2. LoadingFinished — the only message Bug 310251 taught to carry a sourceMappingURL — is never sent for a memory-cache hit.

3. The SI memory-cache path is also thinner than the legacy path. Legacy InspectorNetworkAgent builds a full Network.CachedResource protocol object (which has a sourceMapURL field) and ships it. ProxyingNetworkAgent::requestServedFromMemoryCache instead synthesizes a fake requestWillBeSent + responseReceived and stops there — no CachedResource object and no loadingFinished. So under SI there is currently no field or event on the memory-cache path that can carry a source-map URL to the frontend at all.

A cross-origin iframe CSS file whose source map should auto-link will link on a cold load but silently stop linking once it is served from the memory cache — e.g. after a reload, on back/forward navigation, or when the same stylesheet is reused across frames. Same file, same headers, behavior differs by cache state; in the Network panel the source map simply does not appear for that stylesheet.

[Qianlang Chen]

PR: https://github.com/WebKit/WebKit/pull/68796

[Radar WebKit Bug Importer]

<rdar://problem/181745360>

[EWS]

Committed 316778@main (5ff5b2cf4386): <https://commits.webkit.org/316778@main>

Reviewed commits have been landed. Closing PR #68796 and removing active labels.